Electronics > Beginners
RGB circuit help
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Ian.M:
Problems from left to right:

* S1 connects to wrong end of R1 so it shorts the battery!
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* The combination of T1's Vbe drop and the Vbe drops of T2-T4, means the maximum voltage to the LEDs with S1 open will be Vbatt-2*Vbe.   2*Vbe will be about 1.4V, so even with a fully charged LiPO, there will only be 2.7V at the LEDs.   With the battery discharged to 3.0V there will only be 1.6V at the LEDs.  As the typical Vf of a blue LED is around 3V, it will barely illuminate with a fresh battery.
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* With S1 closed, the drop is Q7's Vce, +  the Vbe drops of T2-T4.  At best that will be somewhere around 1V. At worst, as the 10K resistor starves Q7 for base current, it may not be in saturation, so the drop may be worse than the S1 open case.
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* There are no current limiting resistors for the LEDs.  If the R and G LEDs Vf is less than 2.7V they will probably blow at switch-on with a freshly charged battery.
The *ONLY* way you can make this circuit work prperly is if you use a boost converter to get a higher regulated supply in place of the battery.  Don't forget to add the current limiting resistors.  If you are happy to settle for a total current well under 500mA, probably the easiest option would be to hack a dollar store USB powerbank - that gets you a battery holder for your 18650 cell, a charge controller for it, over-discharge protection and it boosts to 5V for you.   It should be possible to run the LEDs at up to 150mA each, depending on how much the 5V output droops with a 450mA load.
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